This invention relates to inertial angular rate sensing devices and systems. More specifically, the invention relates to rate sensing systems utilizing fluid as a power source.
Many vehicles or platforms (e.g., vehicle gun platforms, radar antennas, aircraft flight control systems) utilize rate sensing devices in closed loop control systems to stabilize their orientation in inertial space as they are exposed to external disturbances such as wind gusts or terrain changes. In addition, many aircraft and missiles are aerodynamically unstable, requiring rate feedback using rate sensors to make them stable or flyable by a human pilot.
There are a variety of gyroscopes which utilize optics or spinning or vibratory masses to sense angular rate. The resulting signal is converted to an appropriate electrical signal, processed by an electronic controller, and transduced to a mechanical force or position to move a valve in the hydraulic actuators driving the vehicle's control surfaces. Although these electronics-dependent rate control techniques ar highly developed and have many applications, there are many drawbacks:
(1) Expense in converting the rate signal from one medium to another (i.e., mechanical to electronic to mechanical to fluid).
(2) Lack of reliability. Aircraft flight controls typically require more than one channel of redundancy to provide the required degree of safety.
(3) Dependency on electrical power.
(4) Lack of ruggedness of gyroscope technologies makes them susceptible to damage from gun shock, handling, and other extreme environments.
(5) Low level electronic signals are susceptible to electromagnetic interference.
(6) Electronic devices are subject to permanent damage by lightning strike or energy beam weapons no under development.
This creates the need for a primary or back up angular rate sensing system that does not require electrical power or electronic signal processing--ideally a sensing and control system which utilizes the actuation fluid supply as the sensor power source also
There has been a great deal of previous development work on inertial angular rate sensors which utilize fluid as a power source. Most work has centered on the vortex angular rate sensor and the laminar jet angular rate sensor. The vortex angular rate sensor detects the change in angular momentum in fluid spinning in a vortex chamber as the chamber is rotated in inertial space. The laminar jet angular rate sensor consists of a nozzle, a laminar jet issuing from the nozzle, and a pair of ports located symmetrically about the jet center line. The laminar jet angular rate sensor concept is considered to be superior to the vortex type in linearity, signal-to-noise ratio, fluid power consumption, ease of manufacture, and compatibility with fluidic amplifier techniques. The primary drawback in prior art angular rate sensors is the variation in critical performance factors such as gain and zero, or null offset, as supply fluid viscosity changes with temperature. This is especially a problem with hydraulic fluid, which can exhibit a 2 to 1 viscosity change with only 10.degree. to 20.degree. F. temperature change.
To obtain acceptable signal-to-noise ratio from the sensor, it is operated in the fully laminar flow regime. Previous testing shows the sensor to be laminar for a Reynold's number (R.sub.e) less than 1200 ##EQU1## .rho. is the supply fluid density .mu. the supply fluid viscosity
V is the jet spouting velocity and PA1 d is the jet depth.
The sensor gain is highest when operated at the maximum Reynold's number which still provides laminar flow. Previous attempts at applying the laminar jet angular rate sensor have either utilized a fixed supply pressure or have varied supply pressure to keep the jet Reynold's number in a constant 900 to 1100 range. The Reynold's number control technique keeps jet spouting velocity proportional to viscosity. For a short nozzle, this requires a differential pressure across the nozzle nearly proportional to the square of viscosity changes. This can greatly reduce null offset and prevent transition to turbulence, but the resulting large pressure changes still allow large viscosity related variations in gain, the most important sensor characteristic in a vehicle rate stabilization system.